Spontaneous recovery from hepatitis B (HBV) and C (HCV) infections is associated with vigorous virus-specific CD4 and CD8 T cell responses that we have previously shown to remain detectable for decades after recovery. In the current studies, we asked whether successful antiviral therapies, i.e. lamivudine therapy for hepatitis B and peginterferon/ribavirin therapy for hepatitis C, result in the same type of immunological T cell memory as spontaneous recovery.[unreadable] [unreadable] As regards to HBV infection, we have identified and characterized novel CD4 T cell epitopes within the HBV polymerase and used them to analyze the immunological effects of long-term antiviral therapy as compared to spontaneous recovery from HBV infection. All epitopes bound with high affinity to the most prevalent HLA-DR antigens, were conserved among HBV genomes and stimulated in vitro recall IFN-gamma responses of in vivo-primed HBV-specific CD4 T cells. Several epitopes contained nested MHC class I motifs and stimulated HBV-specific IFN-gamma production and cytotoxicity of CD8 T cells in addition to CD4 T cell responses. As expected, HBV polymerase-specific responses were more frequent during acute, self-limited hepatitis B and after recovery from HBV infection (12/18, 67%) than during chronic hepatitis B (16/48 (33%); p=0.02). Antiviral therapy of chronic patients restored HBV polymerase and core-specific T cell responses during the first year of treatment, but thereafter responses decreased and, after 3 years, were no more frequent than in untreated patients. The data provide a rationale for the combination of antiviral (lamivudine) therapy with immunostimulatory therapies. The newly described HBV-polymerase epitopes could be a valuable component of a therapeutic vaccine for a large and ethnically diverse patient population. [unreadable] [unreadable] As regards to HCV infection, we have prospectively studied HCV-specific T cell responses of 9 patients who cleared HCV spontaneously after acute infection and of 42 patients who responded to treatment in the acute (n=12) or chronic phase of infection (n=30). T cells were tested for proliferation and IFN-gamma production in response to HCV proteins and 600 overlapping peptides spanning the complete HCV sequence.[unreadable] As expected, all patients with acute hepatitis C displayed vigorous and multispecific IFN-gamma production by CD4 and CD8 T cells. Surprisingly, however, HCV-specific T cell responses decreased after initiation of therapy in all treatment responders and proliferative responses to HCV proteins remained significantly weaker than in spontaneously recovered patients (mean stimulation indices 7.1 versus 25.8 respectively; p less than 0.05). Proliferation and IFN-gamma-production of HCV-specific memory T cells after treatment-induced recovery from chronic hepatitis C resembled those after treatment-induced recovery from acute hepatitis C and were significantly weaker than those of spontaneously recovered patients. [unreadable] These data suggest qualitative differences in function and/or differentiation of HCV-specific memory T cells after treatment-induced and spontaneous HCV clearance that are currently under investigation at the single cell level. Because the relatively weaker memory T cell response of treatment-recovered patients may affect the degree of immune protection upon reexposure to HCV, the data provide a rationale for the combination of antiviral and immunostimulatory therapy.[unreadable] [unreadable] The development of immunostimulatory therapies is hampered by the fact that most vaccines are recombinant subunit vaccines. Subunit, i.e. protein vaccines generally elicit good humoral immune responses, but only weak helper T cell responses and no cytotoxic T cell responses because they do not readily cross into the MHC class I processing pathway. In an effort to identify adjuvants that enhance cellular immune responses to subunit vaccines, we evaluated the quality and strength of CD4+ and CD8+ T cell responses induced by the novel AS02A adjuvant, which consists of monophosphoryl lipid A (MPL), QS21 and an oil-in-water emulsion. Two groups of 15 healthy adult volunteers were vaccinated with either recombinant HBV protein (containing the small protein and the modified large protein of the HBV envelope, including pre-S1 and pre-S2 sequences) adjuvanted with AS02A or with non-adjuvanted HBV protein. [unreadable] Using a pool of overlapping 20-mer peptides a cytotoxic CD8 T cell response was detected in 6 out of 14 HLA A2-positive and -negative recipients of the adjuvanted vaccine. All HLA A2- positive subjects in the adjuvanted group and up to 30% of the subjects in the nonadjuvanted group displayed a cytotoxic CD8 T cell response against selected HLA A2-restricted CD8 T cell epitopes. The adjuvanted vaccine also induced fast and vigorous humoral and CD4 T cell responses of the T helper 1 type. AS02A can therefore be considered as a useful adjuvant that strongly enhances the CD4 and CD8 T cell response to subunit protein vaccines.